The present invention relates to a flat (planar) antenna of the dipole type which is arranged to be supplied with an electrical signal, and/or to produce an electrical signal at one end.
An antenna of the type to which the present invention relates may be contained in a transponder of a "passive interrogator label system".
A "passive interrogator label system system", so-called, is a radar system utilizing transponders which are capable of receiving an interrogating first signal, processing this signal and transmitting a second signal in reply that is derived from the first signal and contains encoded information. Because the encoded information normally includes an identification code that is unique to each transponder, and because the transponders of such a system are relatively light weight and small and may be easily attached to other objects to be identified, these transponders are sometimes referred to as "labels". Furthermore, the transponders, which may be implemented by SAW devices, carry no self-contained power source, such as a battery, that must be periodically replaced. Consequently, these transponders are denominated as being "passive"; hence the name "passive interrogator label system".
Passive interrogator label systems of the type to which the present invention relates are disclosed, for example, in the following U.S. patents:
U.S. Pat. No. 4,737,789 of Paul A. Nysen for "Inductive Antenna Coupling for a Surface Acoustic Wave Transponder";
U.S. Pat. No. 4,703,327 of Anthony J. Rossetti and Paul A. Nysen for "Interrogator/Receiver System for Use with a Remote Transponder"; and
U.S. Pat. No. 4,737,790 of Halvor Skeie and Donald Armstrong for "Passive Interrogator Label System with a Surface Acoustic Wave Transponder Operating at its Third Harmonic and Having Increased Bandwidth."
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one passive transponder which receives this first signal, processes it, and sends back a second radio frequency signal containing encoded information; and a receiver, normally located next to the interrogator, for receiving the second signal and decoding the transponder encoded information.
The passive transponder, as disclosed in these patents, comprises a SAW device and a dipole antenna, which is either electrically or inductively coupled to the SAW transducers on the SAW device. The dipole antenna may be formed either by printing conductive ink (e.g. silver ink) on a substrate in the prescribed antenna pattern, or by stamping a metal foil into the size and shape of the antenna pattern, and bonding this foil by heat and pressure to the substrate (e.g. a polyethyline coated Mylar sheet). The antenna with its substrate is therefore relatively flat and thin and, depending upon the radio frequency of operation, is relatively modest in its lateral dimensions.
For example, the antenna characteristics required for this application, operating at a frequency of approximately 915 MHz., determine that the antenna be physically approximately one inch wide and three inches long. The SAW device, to which the antenna is coupled, is also placed on the substrate to form the complete assembly or "label".
Whereas the dipole antenna, with the SAW device mounted between the dipoles, is satisfactory for most ID tag applications, it would be advantageous, in some applications, to locate the SAW device at one end of the antenna. Such arrangement would simplify packaging the SAW device--antenna combination (label) and would reduce the effect on the label of an adjacent physical body.